Across Antarctic, other glaciers hold back 4 meters of sea level rise

Once they start melting, the oceans would rise for thousands of years.

A glacier isn’t the kind of thing you’d expect to get away from you. After all, only the world’s fastest-flowing glaciers can match a snail’s pace. But we know it’s possible for glaciers to have tipping points that, once crossed, result in an unstoppable change. Once unstable, they can lose a lot of ice before finding another stable configuration.

Looking back through the history of the Antarctic ice sheets, we know that they have been susceptible to warming in the past. The West Antarctic Ice Sheet is especially vulnerable because a great deal of the continent beneath it is below sea level. If the ice shrinks back from the higher elevation areas, the entire ice sheet can collapse, as it may have done several times in the last million years. Some of the West Antarctic glaciers that prevent this collapse have behaved dynamically in the recent past—and, as we saw this week, there's evidence that we may be committed to seeing a repeat performance.

Further Reading

There's little that will stop continued retreat of Antarctic glaciers.

The amount of ice present there today could raise global sea level roughly several meters if it all melted. But across the continent, the East Antarctic Ice Sheet is much larger, holding the equivalent of 55 meters of sea level rise as ice. Fortunately, it's perched securely above sea level. Researchers are less concerned with the potential for tipping points there. There are, however, exceptions. Some East Antarctic glaciers havemelted back considerably in the past. The key is to figure out how much ice they can lose and how fast they can lose it.

A new study from Potsdam Institute for Climate Impact Research scientists Matthias Mengel and Anders Levermann focuses on the Wilkes Basin portion of the East Antarctic Ice Sheet. This large basin is situated just across the Transantarctic Mountains from the Ross Sea. Its ice flows seaward through the Ninnis and Cook ice streams. The topography beneath these glaciers drops down as you head inland along a pair of major valleys.

That increase in depth creates a classic tipping point for glaciers that reach the sea. Many factors affect the flow of that ice, including the friction between the glacier and the ground below. If a glacier melts back into a deeper spot, the water pressure at the base increases, offsetting some of the glacier’s weight. That means less resistance to flow, and the glacier sheds mass more and more rapidly. This can continue until the front of the glacier retreats to another high spot and reestablishes a stable interface with the ocean. The problem is that the next high spot may be many kilometers inland.

To find out what that kind of behavior might mean for the Wilkes Basin, the researchers used a computer model. Warming ocean water has been observed beneath the floating ice shelves in front of glaciers a little further to the west, so the researchers simulated similar warming in the model. In various scenarios, they increased the ocean water temperature by 1–2.5°C and maintained it for two, four, or eight centuries before dialing the temperature back down.

The simulated glaciers crossed a significant tipping point. With a couple of centuries at the warmer end of the temperature range, or four centuries at the lower end, the glaciers continued shrinking long after the ocean water returned to its initial temperature. And not just a little. By the time the glaciers found a new stable position (on the other side of the valleys they had retreated into), they had lost enough ice to raise global sea level three to four meters.

That situation took about 25,000 years to play out (with most of the ice lost in the first 10,000 years), meaning the peak melt rate was about double Antarctica’s present-day total contribution to sea level rise. Still, once past the tipping point, it retreated some 700 kilometers all on its own—it wasn’t forced to by continued warming.

The researchers describe the relatively small amount of ice that has to disappear to set this in motion as a “plug." Melt that plug, driving the front of the glacier off the high ground on which it currently sits, and a huge volume of ice drains out as the glacier retreats to the next high ground. Once the plug is pulled, there’s no sticking it back in.

While this is unlikely to have a large impact on sea level by, say, the end of this century, it tells us about the kind of long-term change we could commit ourselves to as the climate warms. Turning to the past, the researchers note that the Wilkes Basin might explain where some of the water in previous sea level rise events came from. It's easy to get information about how high the oceans rose, but it takes more work to figure out where ice was lost to make that happen. The Wilkes Basin could have chipped in a lot.